The present invention generally relates to antennas and, more particularly, relates to electronically scanned antennas with reconfigurable dipoles and an associated method of operation.
Radar and communication systems require antennas to transmit and receive electromagnetic (EM) signals, generally in the microwave or millimeter wave spectrum. One class of antennas is the electronically scanned antenna (ESA). In an ESA, the signal is transmitted and received through individual radiating elements distributed uniformly across the face of the antenna. Phase shifters in series with each radiating element create a well-formed, narrow, pencil beam and tilt its phase front in the desired direction (i.e., xe2x80x9cscanxe2x80x9d the beam). A computer electronically controls the phase shifters. ESAs offer fast scan speeds and solid state reliability.
While ESAs have proven effective in many applications, the main deterrent to their widespread application is their high cost. Another drawback is that ESAs have higher insertion losses associated with their phase shifters than mechanically scanned antennas. These losses increase the output power required of the transmitter of the ESA which, in turn, increases its cost, power supply requirements and thermal management due to the increased power dissipation.
One approach to overcome the aforementioned loss issued is the use of an active ESA (AESA). The AESA is constructed by pairing amplifiers with phase shifters in the antenna. An AESA incorporates a power amplifier to provide the requisite transmitted power, a low noise amplifier to provide the requisite receiver sensitivity and a circular connecting the transmit and receive channels to the radiating element. This approach is viable for small arrays, i.e., arrays of a few hundred elements. But for a given antenna size, the number of radiating elements increases as the square of the frequency. Thus, for a high gain, millimeter wave antenna, the array often contains thousands of elements. In such an instance, cost, packaging, control, power distribution and thermal management issues become significantly important concerns.
Space-fed configurations using a passive ESA (PESA) promise to be less expensive than AESAs for millimeter wave applications. A PESA does not use distributed amplifiers, but instead relies on a single high power transmitter and a low loss antenna. The reason for the lower cost is the simpler, space-fed architecture of such an antenna that has fewer, less expensive parts. A PESA can be implemented in a number of quasi-optic configurations such as a focal point or offset J-feed reflection antenna, as a transmission lens antenna, as a reflection Cassegrain antenna, or as a polarization twist reflection Cassegrain antenna. But since PESAs do not have amplifiers to overcome the circuit losses, such losses, and particularly the phase shifter losses, become a key issue.
One approach to reduce phase shifter insertion loss is to implement the phase shifter with a micro-electromechanical system (MEMS) switch. The MEMS switch can be employed as the control device in various types of phase shifter designs. Since it has an electromechanical switch, it offers low insertion loss. A microwave monolithic integrated circuit (MMIC) of MEMS-based phase shifters and radiators can be fabricated as a sub-array. This scale of integration promises lower costs. But MEMS-based MMIC phase shifters remain expensive and their integration into a full array will be even more costly for a millimeter wave antenna. They are also relatively fragile compared to solid-state devices and require high control voltages, such as 70 Volts. For some configurations, packaging the phase shifter and radiator(s) in the requisite cell area, the maximum area that the radiating element can occupy for proper operation over a given maximum frequency and scan angle, is also difficult.
In view of the foregoing background, the present invention provides an improved electrically scanned array antenna and a method of forming the same. According to embodiments of the present invention, the antenna includes an array of array elements that can be controlled to thereby impart a desired degree of phase shift to an electromagnetic signal received thereon. Advantageously, this is accomplished without the need for any electromechanical phase shifters. Also, the array can be formed as a single layer including the array elements formed on a substrate. As such, the array can be less complex and can be less expensive to fabricate, when compared to more conventional multi-layer antenna designs. In addition, for an electronically scanned antenna fabricated according to embodiments of the present invention, the wafer costs will be less than a Gallium Arsenide (GaAs) wafer used in MEMS/MMIC phase shifters. It is also more amenable to large wafer sizes that can accommodate an entire array in a single wafer. In this regard, the construction of the antenna according to embodiments of the present invention is less complex and may exhibit less loss than MEMS/MMIC technology.
According to one aspect of the present invention, an electronically scanned antenna system includes a plurality of array elements and a control system. Each array element has a plurality of reflecting components, such as resonant cross dipoles, capable of reflecting an electromagnetic wave incident thereon. Each reflecting component, in turn, is interconnected to at least one reflecting segment, such as a dipole segment, by at least one switch, such as a transistor. In this regard, when the switches are in a closed state the respective reflecting segments are electrically coupled to the respective reflecting components to thereby alter a reflective geometry of the respective reflecting components. The control system is capable of controlling the switches to thereby control the array elements to provide a desired degree of phase shift to a signal reflected from the antenna. Advantageously, the control system is capable of controlling the switches of one reflecting component at the same time the control system controls corresponding switches of other reflecting components.
More particularly, where the reflecting components comprise resonant cross dipoles and the reflecting segments comprise dipole segments, each resonant cross dipole can comprise two crossing dipole arms, at least one of which is interconnected to a dipole segment of a first length. Also, at least one of the dipole arms is interconnected to a dipole segment of a second length that is shorter than the first length. In one such arrangement, each dipole arm can be interconnected to a dipole segment of the first length on one end and a dipole segment of the second length on an opposing end. The antenna can therefore provide a first degree of phase shifting to a received electromagnetic signal when the dipole segments of the second length are electrically coupled to respective dipole arms.
In addition to being interconnected to a dipole arm by a switch, at least one dipole segment of the second length can be interconnected to another dipole segment by another switch on an opposing end. In such an arrangement, the antenna can provide a first degree of phase shifting to a received electromagnetic signal when the dipole segments of the second length are electrically coupled to respective dipole arms. In addition, the antenna can provide a second degree of phase shifting to a received electromagnetic signal when the dipole segments of the second length are electrically coupled to respective dipole arms and the other dipole segments are electrically coupled to respective dipole segments of the second length. Further, the antenna can provide a third degree of phase shifting to a received electromagnetic signal when all of the dipole segments are electrically coupled to respective dipole arms, and all of the other dipole segments of the second length are electrically coupled to respective dipole segments of the second length.
An electrically scanned antenna and method of forming the same are also provided.